U.S. Pat. No. 5,024,711 of Gasser et al discloses a method for preparing a surface for bonding with a synthetic resin. In the method, a layer is supplied onto a substrate surface by sandblasting with a composition comprising an optionally silanized material having a particle size less than 5 microns having a hardness greater than that of the substrate surface, and/or a silanized siliceous material having an average particle size of 2-200 microns, and the remainder of the sandblasting composition having an average particle size greater than 5 microns. There is not disclosed in U.S. Pat. No. 5,024,711, the use of an activator which can catalyze or accelerate the hardening of an activatable adhesive.
Adherence between substrate surfaces, e.g. made of wood, glasses, metals, ceramic or plastic materials, by means of plastic-containing adhesives or the coating of substrates with plastic materials should be permanent under even the most varied of weather conditions. Moreover, it is often desired that the adhesive strength between the substrate and a plastic piece bonded thereto, even under the most varied of stresses (such as mechanical or thermal deformations) should ideally always be stronger than the inner strength (cohesive strength) of the plastic material itself.
A process for preparing a substrate surface for bonding with plastic materials is known from EP-A-0 326 097, which is expressly incorporated herein by reference. In the process, a substrate surface is coated by sandblasting with a sandblasting material consisting of,
(A) 0.01 to 90% by weight of an optionally silanized material having a particle size of &lt;5 .mu.m and a hardness greater than the hardness of the substrate surface, and/or PA1 (B) 20 to 100% by weight of a silanized, silicon containing material having an average particle size of 2 to 200 .mu.m; and PA1 (C) the remainder being a sandblasting agent having an average particle size of &gt;5 .mu.m;
the percentages above being based upon the weight of the sandblasting material. The thus obtained adhesion-promoting layer may optionally be silanized. A similar process is also known from DE-B-38 02 043, which is also incorporated by reference herein.
The above two processes increase the adhesive strength of adhesives on substrates and the long-term stability of the bonding therebetween considerably; but they are not suitable for use with, for example, anaerobic adhesives as their polymerization and bonding can only be initiated by catalysis (e.g., metal ions) with exclusion of air. In this regard, the thin inorganic layer being applied in both of the above processes completely or largely prevents the polymerization of anaerobic adhesives. Other "activatable" adhesives, for example, cyanacrylate adhesives can also only give poor adhesion results with the aforementioned processes.
In such cases, as with other non-metallic substrate surfaces (e.g. plastic materials), the polymerization of the product can only be initiated or accelerated by applying an activator, which in the instance of anaerobic adhesives usually contain metal compounds which readily release ions (cf. G. Habenicht in "Kleben", Springer Verlag, 1986, p. 15ff, particularly p. 18).
However, the adhesive strength obtained in this way has been unsatisfactory.